self-full-filling.js

function selfFullFilling(fn) {
  let value;
  return function() {
    return value || (value = fn.apply(this, arguments));
  };
}

const add = selfFullFilling(function(a, b) {
  console.log("calculated!")
  return a + b;
});

add(4,5) // returns 9 … logs "calculated"
add(4,5) // only returns 9